In the prior art, various heat transfer methods have been known. Among them, there has been proposed a method in which a sublimable dye is used as the recording material, which is carried on a substrate sheet such as a paper or a plastic sheet to provide a heat transfer sheet. Various full-color images are formed on an image receiving sheet dyeable with a sublimable dye, for example, an image receiving sheet having a dye receiving layer provided on the surface of a paper or a plastic film.
For this method, a thermal head of a printer is used as the heating means, and a large number of color dots of 3 colors or 4 colors are transferred onto an image receiving sheet by heating for a very short time, thereby reproducing the full-color image of the original by color dots of multiple colors.
The image thus formed is very sharp and also excellent in transparency, because the colorants used are dyes, and therefore the image obtained is excellent in reproducibility and gradation of the intermediate color, and is similar to an image obtained by conventional offset printing or gravure printing. It has also become possible to form an image of high quality comparable with full-color photographic image.
The heat transfer image receiving sheet to be used in the sublimation type heat transfer system, when a reflected image is required as in the printed matters or photography, one having a dye receiving layer comprising a resin having good dyeability provided on the surface of an opaque substrate sheet such as paper or synthetic paper is used, while when a light-transmissive image is required as used in OHP (overhead projector), one having a dye receiving layer provided on a transparent substrate sheet such as polyester film is used.
In any case, when image formation is effected by use of these image receiving sheets, as the result of heating of the surface of the image receiving sheet during transfer, warping (curl) tends to occur on the image receiving sheet, whereby slippability of the image receiving sheet after printing becomes worse and causes the problem of paper jamming. Also, since it is necessarily unavoidable to heat the whole printer to a certain temperature, curl is generated within the printer also before printing, whereby conveyability of the image receiving sheet is worsened to generate a problem such as double delivery, etc.
Further, in the case of forming a multi-color image, since printing is performed for 3 to 4 times on the same image receiving sheet, curl becomes excessive and generates non-matching of the printed dots and lower image quality. Particularly in the case of a light-transmissive image for OHP, a lowering in the image quality becomes further marked, because the printing image is projected with enlargement to several fold.
The present invention has been accomplished in view of the above-mentioned problem. The object of the present invention is to provide an image receiving sheet, which does not curl due to the heat applied thereto by a thermal head during image transfer, can make delivery of the sheet better during image transfer, and will not give rise to color slippage or deformation of the image transferred on the receiving layer.